For example, the United State has promised countries forsaking nuclear weapons that they will not be targeted by U.S. nuclear warheads. Yet classified presidential guidance (PDD 60) reportedly leaves open the possibility of retaliation . or even a first strike . against a threat or attack by chemical/biological weapons. U.S. Air Force doctrine based on this guidance states, "Because the United States lacks the ability for an in-kind response to chemical and biological weapons, it must maintain a credible nuclear deterrent against all forms of WMD."